IDENTIFICATION OF A NOVEL FORM OF THE ALPHA(3) INTEGRIN SUBUNIT - COVALENT ASSOCIATION WITH TRANSFERRIN RECEPTOR

The alpha(3) beta(1) integrin is a cell-surface receptor for laminin, entactin, collagen, fibronectin and epiligrin. On some prostatic-carcinoma cell lines that express the alpha 3 beta(1) heterodimer we have identified a novel form of the alpha 3 subunit. Whereas the prototypic alpha 3 subunit has a molecular mass of similar to 155 kDa, we have isolated a similar to 225 kDa protein (p225) which is recognized by monoclonal antibodies to the alpha(3) subunit. Protein sequence analysis revealed that p225 consists of two polypeptides, namely integrin alpha(3) heavy chain (similar to 130 kDa) disulphide-bonded to a monomer of the transferrin receptor (similar to 95 kDa) instead of the typical alpha(3) light chain (similar to 25 kDa). The p225 seems to be directly associated with beta(1)subunit, since it was immunoprecipitable with anti-(beta 1subunit) antibodies. The association of transferrin receptor and integrin alpha(3) was apparently not the result of spurious disulphide-bond formation occurring during the protein purification, as iodoacetamide and GSH did not block the formation of the complex. The transferrin receptor is normally a homodimer that is involved in the internalization ofiron-bound transferrin into cells and can be expressed at relatively high levels in the cell lines which we have studied. The p225 is not found on all cell types examined to date and therefore it may represent a unique complex between the integrin alpha(3) subunit and the transferrin receptor, a covalent association which may play a role in the adherence and/or proliferation of some types of tumour cells.